Optimal timing of the second trimester fetal ultrasound in the obese gravida

Abstract

Objectives: Obesity is increasingly prevalent in the obstetric patient population, becoming one of the most commonly occurring risk factors in obstetric practice. Obesity has implications for maternal and fetal morbidity; in fact, some data suggest it is associated with higher rates of fetal anomaly. Coincident with this, maternal obesity poses an inherent challenge for ultrasound quality. The objective of this study is to investigate the relationship between ultrasound completion rates and advancing gestational age in obese gravidas, and to help identify an ideal gestational age to perform the second trimester ultrasound for fetal evaluation in the obese patient population. We hypothesized that in the obese patient, the odds of a completed scan would increase with each gestational age week, as fetal size increases.

Methods: This is a retrospective cohort study at a single tertiary care center. Inclusion criteria were pregnant women with BMI greater than 30 who had second trimester fetal ultrasound and delivery at our institution. Exclusion criteria were pregnancies without documented BMI, ultrasound not performed between 18 0/6 and 21 6/7 weeks, multiple gestations, or ultrasound performed for indication other than fetal anatomic assessment. Ultrasound reports were considered incomplete if they indicated suboptimal or non-visualization of any anatomic structures included in the comprehensive anatomic survey. Demographic data was compared using Student's t test and chi-square analysis where appropriate. Chi-square analysis was used to compare rates of completed surveys. p < .05 was considered significant.

Results: After application of eligibility criteria, our cohort included 1,954 subjects. When comparing subjects with a completed scan to those with an incomplete scan, there were more white subjects in the incomplete group (p = .012), but other analyzed demographics were similar between groups. When using 18 weeks as a referent group, with each additional completed week of gestation, subjects were more likely to have a completed scan, at 19 weeks (OR 1.29, CI 1.05-1.58); at 20 weeks (OR 1.46, CI 1.1-1.95); at 21 weeks (OR 2.12, CI 1.42-3.17) (p < .05 for each). This association persisted when adjusting for demographic factors. To identify the optimal timing for the second trimester ultrasound for fetal evaluation, we re-analyzed the data using each completed week of gestational age as the referent group. When using 19 weeks as the referent group, the odds of a complete scan were lower at 18 weeks, and higher at 21 weeks, but not different at 20 weeks. When using 20 weeks as the referent group, the odds of a complete scan were lower at 18 weeks, but not different at 19 or 21 weeks. Finally, when using 21 weeks as the referent group, the odds of a complete scan were lower at 18 and 19 weeks, but not different at 20 weeks.

Conclusions: In this cohort of obese gravidas, the odds of having a completed anatomic survey continued to improve until 21 weeks gestational age. When comparing completed scans between each week, 18 weeks demonstrated consistently lower odds of a complete scan, however 20 weeks did not differ significantly from 19 weeks. Therefore, consideration should be made to perform the initial second trimester ultrasound for fetal evaluation in obese patients at 19 weeks gestational age to optimize completion rates while minimizing scans performed at advancing gestational ages.

Keywords: Ultrasound timing; anatomic survey; obese; pregnancy; second trimester.